1. Field of the Invention
The present invention relates to a recording medium for recording information by varying optical characteristic thereof by application of an external energy thereto.
2. Description of the Related Art
The recording media of this type are, for example, optical recording media which can be represented by a recordable compact disk (i.e., CD-R). In general, the CD-R comprises a substrate, a recording layer (or light absorbing layer) disposed on the substrate, and a reflection layer further disposed on the recording layer. The recording layer includes, for example, an organic pigment. The reflection layer includes, for example, gold or silver. The recording layer is deformed or modified by applying a laser beam as an external source, thereby varying the optical characteristic of the deformed or modified portion and recording information thereon. Then, the information is reproduced optically by utilizing the reflection difference between the recorded portions and unrecorded portions of the recording layer.
However, the above conventional recording layer is formed of organic materials. Accordingly, in view of the costs of the organic materials themselves and the material loss in the manufacturing process, the conventional recording layer suffers from the high manufacturing cost.
In order to solve the problems, there have been several proposals. According to the proposals, inorganic materials which are less expensive than organic pigments are employed to form the recording layer by good cost-performance methods, such as a vapor deposition process and a sputtering process. There have been proposed the following optical recording media employing the inorganic-material recording layers: for example; metal sulfide is used to form the inorganic-material recording layer as set forth in Japanese Unexamined Patent Publication (KOKAI) No. 4-193,586; and sulfar or selenium is used to form the inorganic-material recording layer as set forth in Japanese Unexamined Patent Publication (KOKAI) No. 2-152,029.
The inventors of the present invention investigated into the recording media which employed the inorganic-material recording layers disclosed in the publications. As a result, they found out that the recording media suffered from the following drawbacks in terms of environmental resistance and information retention.
Specifically, the inventors examined the environmental resistance of the recording layer which was made from metal sulfide, for instance, SnS, and on which information was recorded by a laser beam. When the recording layer was subjected to a high-temperature-and-high-humidity condition (for example, at a temperature of 90.degree. C. and in a relative humidity of 80% RH) for a long period of time (for instance, for 240 hours), the recording layer was humidified and swollen to alter. Thus, the information recorded on the recording layer could not be reproduced.
Moreover, in the recording layer employing sulfur, the sulfur was reacted with a metal (for instance, silver), which was included in the reflection layer of the recording medium, to form sulfide (for example, silver sulfide) by using a laser beam. Thus, the optical characteristic of the recording layer is varied to record information thereon. However, the reaction developed gradually to form the silver sulfide when the recording layer was left even at room temperature, because the reactivity of the reaction was high. As a result, the recording layer retained the information unstably.